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irregular satellites

…the Sun that were gravitationally captured by their respective planets. Neptune’s moon Triton and Saturn’s Phoebe are prominent examples of captured moons in retrograde orbits, but every giant planet has one or more retinues of such satellites.

Nereid

… is an object that was captured by Neptune’s gravity and whose billion-year-long “settling-in” process severely disrupted Neptune’s original system of moons. On the other hand, Nereid itself may be a captured object that formed elsewhere in the solar system.

origin of Earth-Moon system

According to capture theories, the Moon formed elsewhere in the solar system and was later trapped by the strong gravitational field of Earth. This scenario remained popular for a long time, even though the circumstances needed in celestial mechanics to brake a passing Moon into just the…

Triton

…the hypothesis that its sibling Triton is an object that was captured by Neptune’s gravity and whose billion-year-long “settling-in” process severely disrupted Neptune’s original system of moons. On the other hand, Nereid itself may be a captured object that formed elsewhere in the solar system.

…is thought to be a captured object that perhaps originally formed, like Pluto, as an independent icy planetesimal in the outer solar system. At some point in Neptune’s early history, Triton’s orbit around the Sun may have carried it too near the giant planet. Gas drag in Neptune’s extended atmosphere…

…suggests that it is a captured object, which perhaps formed originally, like Pluto, as an independent icy planetesimal in the outer solar system’s Kuiper belt. Its original orbit would have been highly eccentric, but tidal interactions between Triton and Neptune—cyclic deformations in each body caused by the gravitational attraction of…